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When a bridge fell in Washington last month it rekindled questions about the state of our nation's infrastructure.

That span over the Skagit river had been rated both "functionally obsolete" and "fracture critical" prior to collapse. Right now in California there are more than 600 bridges labeled either functionally obsolete, fracture critical or "structurally deficient."

The structurally deficient rating means that a bridge has a physical flaw of some kind. That could be something serious, but in most cases it's just cracked pavement or peeling paint.

Functionally obsolete means a bridge isn't up to current building standards. That often means the lanes are narrow or there is no shoulder for stalled cars. Sometimes it denotes a more serious structural issue that will force Caltrans to repair or rebuild a structure.

Fracture critical means a bridge doesn't have a secondary support system to stop it from collapsing if it is seriously overloaded, or if it experiences a significant accident that impacts a vulnerable section. Both the bridge in Washington state and the one that fell in Minnesota in 2007 were rated fracture critical.

"This kind of accident is always [a] very small probability thing," said UCLA structural engineering professor Jian Zhang. She maintains these circumstance are very rare, and that a fracture critical label does not necessarily mean a bridge is structurally unsound.

Fot its part, Caltrans says it shuts down any bridge that's deemed unsafe.

The agency recognized that the ominous-sounding federal classifications don't show the true quality of a bridge. So the state developed its own rating system, called the Bridge Health Index.

Rather than giving a general sense of how a bridge is doing, like the federal system does, the Bridge Health Index drills down into specifics. It includes inspection of more than 100 components of a structure, from the columns to the girders to the guard rails. Using this system, Caltrans can not only understand how sturdy a bridge is, but which parts might need work or replacement.

Next year, the federal government plans to enact a similar system.

Still, not everyone is satisfied with this, or with any visual inspection method.

"There could be a huge crack on the inside of a multi-ton structural member, except it hasn't become visual yet for three more years," LePatner argued.

In the future he would like to see bridges fitted with sensors that can detect structural problems long before they appear. In fact, some bridges in California already use this kind of system, like the Vincent Thomas Bridge in Long Beach.

For the most part, though, states still rely on human inspectors to gauge the health of a bridge.